The broad, long-term objective of this research plan is to understand the basis of protein-protein interactions. The focus is on coiled-coil interactions, found in the "leucine-zipper" class of DNA-binding proteins. Extension of these studies to the antiparallel coiled coil found in a tRNA synthetase and to the "helix-loop-helix" class of transcription factors is also proposed. The methodology involves structural dissection of proteins into modules that display the essential features to be examined. The specific aims of the research are: (1) To evaluate the contributions to stability of H-bonded ion pairs (i.e., salt bridges) in the crystal structures of the dimer, trimer and tetramer of the leucine zipper from GCN4 (a yeast transcriptional activator), in order to understand the factors that determine the strengths of electrostatic interactions in proteins. (2) To characterize the solution properties of various GCN4 leucine-zipper peptides, with numerous techniques including nuclear magnetic resonance (NMR) spectroscopy and amide proton exchange, in order to understand the determinants of uniqueness in protein structures. (3) To determine the mechanism for antiparallel coiled-coil formation by peptides that correspond to a segment of the seryl tRNA synthetase, in order to understand the factors that influence parallel versus antiparallel orientation of alpha-helices. (4) To extend our studies of the transcription-factor modules to the helix-loop-helix (HLH) motif, which is sometimes found adjacent to the leucine-zipper region of transcription factors, including the nuclear oncoproteins Myc and Max. The interaction between proteins is central to much of physiology. Many of the protein fragments that are studied here are derived from medically important proteins. As a result, these studies contribute to an increased understanding of transcription, translation, development, and oncogenesis. This work is also directly relevant to some of the central issues in protein folding, which remains a major unsolved problem in molecular biology.